Cyclic AMP-dependent tyrosine phosphorylation in tammar wallaby (Macropus eugenii) spermatozoa

Fibronectin stimulates human sperm capacitation through the cyclic AMP/protein kinase A pathway

STUDY QUESTION

Does fibronectin (Fn) stimulate the sperm capacitation process in humans?

SUMMARY ANSWER

Fibronectin stimulates human sperm capacitation.

WHAT IS KNOWN ALREADY

Capacitation is a process that occurs in the oviduct. It has been suggested that some molecules present in the oviductal fluid and cells as well as proteins present in the cumulus oophorus could be involved in the modulation of sperm function and their acquisition of fertilizing capacity. Fibronectin is a glycoprotein that is present in the fluid and the oviduct epithelium, and its receptor (alpha 5 beta 1 integrin) is present in human sperm. When alpha 5 beta 1 (α5β1) integrin binds to fibronectin, intracellular signals similar to the process of sperm capacitation are activated.

STUDY DESIGN, SIZE, DURATION

Human sperm were selected via a percoll gradient and were then incubated in non-capacitated medium (NCM) or reconstituted capacitated medium (RCM), in the presence or absence of fibronectin for different time periods. A total of 39 donors were used during the study, which lasted 3 years.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Freshly ejaculated sperm from healthy volunteers were obtained by masturbation. All semen samples were normal according to the World Health Organization parameters. Six approaches were used to determine the effects of fibronectin on sperm capacitation: chlortetracycline (CTC) assay, heterologous co-culture of human sperm with bovine oviductal epithelial cells (BOEC), measurement of cyclic (c) AMP levels, activity of protein kinase A (PKA), phosphorylation of proteins in tyrosine (Tyr) residues, and induction of acrosome reaction with progesterone.

MAIN RESULTS AND THE ROLE OF CHANCE

When sperm were incubated in RCM in the presence of Fn, we observed differences with respect to sperm incubated in RCM without Fn (control): (i) a 10% increase in the percentage of sperm with the B pattern (capacitated sperm) of CTC fluorescence from the beginning of capacitation (P < 0.001); (ii) an effect on both the concentration of cAMP (P < 0.05) and PKA activity (P < 0.05) during early capacitation; (iii) an increase in the degree of phosphorylation of proteins on tyrosine residues after 60 min of capacitation (P < 0.01); (iv) an increase in the percentage of acrosome-reacted sperm in response to progesterone (P < 0.05); and (v) a decrease in the percentage of sperm attached to BOEC (P < 0.05). Moreover, we noted that the effect of Fn was specific and mediated by alpha 5 beta 1 integrin (P < 0.001). Fn by itself had no effect on sperm capacitation.

LIMITATIONS, REASONS FOR CAUTION

This study was carried out with sperm from young adult men. Men with abnormal semen samples were excluded. The results cannot be directly extrapolated to other mammalian species.

WIDER IMPLICATIONS OF THE FINDINGS

Currently, male subfertility has become a huge public health problem, which makes it imperative to develop new treatments. This is a novel discovery that extends our current knowledge concerning normal and pathological sperm physiology as well as events that regulate the process of fertilization.

STUDY FUNDING/COMPETING INTERESTS

This study was supported by grants from FONDECYT (1130341, E.S.D. and 1120056, P.M.) and FONCYT (PIP 2011-0496, S.P.-M). The authors have no conflicts of interest.

Sperm motility activation, sperm heterogeneity and sperm-female tract interactions in Bennett's wallaby (Macropus rufogriseus rufogriseus)

Sperm-oviduct interactions in Bennett's wallaby (Macropus rufogriseus rufogriseus) were investigated using in vitro cocultures of cauda epididymal spermatozoa and oviducal epithelial cells. Kidney epithelial cells were used as non-reproductive control tissues. Spermatozoa attached to epithelial cells of both origins, but sperm survival and activity was higher when cocultured with oviducal cells. New findings during live sperm-epithelial interactions included: (1) a high frequency of reversible head movements, from linear (streamlined configuration) to T shape (thumbtack configuration) in swimming spermatozoa immediately after the start of coculture; (2) the loss of sperm tails (tail shedding) increasing with time; and (3) interrupted swimming patterns, where periods of fast movement were interspersed with slower swimming while the spermatozoa interacted with the epithelial cell surface. Sperm motility activation responses were characterised after diluting the epididymal samples in phosphate-buffered saline, medium M199 and Tyrode's medium. The results confirmed that the marsupial oviduct is able to support the viability and motility of a sperm subpopulation for at least 20 h in vitro and suggest that some spermatozoa shed their tails after binding, possibly as a result of a selective process.

Cyclic 3',5'-AMP causes ADAM1/ADAM2 to rapidly diffuse within the plasma membrane of guinea pig sperm

Because sperm cannot synthesize new proteins as they journey to the egg, they use multiple mechanisms to modify the activity of existing proteins, including changes in the diffusion coefficient of some membrane proteins. Previously, we showed that during capacitation the guinea pig heterodimeric membrane protein ADAM1/ADAM2 (fertilin) transforms from a stationary state to one of rapid diffusion within the lipid bilayer. The cause for this biophysical change, however, was unknown. In this study we examined whether an increase in cAMP, such as occurs during capacitation, could trigger this change. We incubated guinea pig cauda sperm with the membrane-permeable cAMP analog dibutyryl cAMP (db-cAMP) and the phosphodiesterase inhibitor papaverine and first tested for indications of capacitation. We observed hypermotility and acrosome-reaction competence. We then used fluorescence redistribution after photobleaching (FRAP) to measure the lateral mobility of ADAM1/ADAM2 after the db-cAMP treatment. We observed that db-cAMP caused roughly a 12-fold increase in lateral mobility of ADAM1/ADAM2, yielding diffusion similar to that observed for sperm capacitated in vitro. When we repeated the FRAP on testicular sperm incubated in db-cAMP, we found only a modest increase in lateral mobility of ADAM1/ADAM2, which underwent little redistribution. Interestingly, testicular sperm also cannot be induced to undergo capacitation. Together, the data suggest that the release of ADAM1/ADAM2 from its diffusion constraints results from a cAMP-induced signaling pathway that, like others of capacitation, is established during epididymal sperm maturation.

Role of the epididymis in sperm competition

Although it is generally understood that the testes recruited kidney ducts for reproductive function during the evolution of vertebrates, little is understood of the biological significance of the adaptation. In the context of the evolution of the mammalian epididymis, this report provides evidence that a major role of the epididymis is to enhance a male's chance of achieving paternity in a competitive mating system. A unique example of sperm cooperation in monotremes is used as evidence that the epididymis produces sperm competition proteins to form groups of 100 sperm into bundles that have a forward motility nearly thrice that of individual spermatozoa. As it required 3-h incubation in vitro under capacitation conditions to release motile sperm from the bundles, it is suggested that the monotremes provide an example of capacitation that is quite different from capacitation in higher mammals. It is suggested that variation between species in the intensity of sperm competition could explain the variation that occurs between species in the amount of post-testicular sperm maturation and storage in the epididymis, an explanation of why the human epididymis does not play as important a role in reproduction as the epididymis of most mammals.

Reactive oxygen species in spermatozoa: methods for monitoring and significance for the origins of genetic disease and infertility

Human spermatozoa generate low levels of reactive oxygen species in order to stimulate key events, such as tyrosine phosphorylation, associated with sperm capacitation. However, if the generation of these potentially pernicious oxygen metabolites becomes elevated for any reason, spermatozoa possess a limited capacity to protect themselves from oxidative stress. As a consequence, exposure of human spermatozoa to intrinsically- or extrinsically- generated reactive oxygen intermediates can result in a state of oxidative stress characterized by peroxidative damage to the sperm plasma membrane and DNA damage to the mitochondrial and nuclear genomes. Oxidative stress in the male germ line is associated with poor fertilization rates, impaired embryonic development, high levels of abortion and increased morbidity in the offspring, including childhood cancer. In this review, we consider the possible origins of oxidative damage to human spermatozoa and reflect on the important contribution such stress might make to the origins of genetic disease in our species.